Recently, many porous materials have been used in many applications for the purpose of various catalysts or of adsorbing or capturing specific volatile organic compounds or gaseous molecules. For this reason, a considerable interest has been concentrated on the technologies concerning various porous materials and their preparation methods.
Such porous materials fall into three classes, inorganic material, organic material and polymer material, depending on the chemical structure and composition of the components. According to the IUPAC (International Union of Pure and Applied Chemistry), those materials are sub-classified based on the size of pores that are introduced in the material: microporous material (pore diameter <2.0 nm), mesoporous material (2.0<pore diameter <50 nm) and macroporous material (pore diameter >50 nm).
Among the porous materials, typical examples of the microporous material are zeolite and carbon nanotube that contain naturally formed minute pores in pore size 2.0 nm or less. Many studies have been made on other porous materials than these natural porous materials and their preparation methods. There are six up-to-date known methods for preparing porous materials.
The first one is a synthesis of mesoporous silica or organic-inorganic hybrid materials using an organic surfactant as a template. The pore size of the product is dependent on the type, chemical structure or reaction conditions of the surfactant (see Nature 2002, 416, 304; Nature 2001, 412, 169; Nano letters 2008, 8(11), 3688; Acc. Chem. Res. 2005, 38, 305; J. Am. Chem. Soc. 2005, 127, 6780; WO09/010945; US Pat. Application No. 20100015026; or WO01/78886).
The second method uses minute metal-organic building blocks of a constant size that are formed by electrostatic attractions between metal cations and organic ligands. These metal-organic building blocks self-assemble into microporous or mesoporous metal-organic frameworks (MOFs) that come in various crystal shapes, including tetrahedral, pentahedral, hexagonal, octagonal, simple cubic, or face-centered cubic (see Nature 1999, 398, 796; Science 2004, 306 1021; Acc. Chem. Res. 2002, 35, 972; WO09/011545; WO06/110740).
The third one involves an organic synthesis method to produce host molecules having a specific pore size, such as crown ether, cryptand and calixarene derivatives, or cyclodextrin. The synthesized host molecules bind to guest molecules fitting into the host cavity to have a function of separating a specific substance (see Angew. Chem. Int'l. Ed. 2005, 44, 2068).
In the fourth method, small organic molecules (so-called porogens) including pentane or toluene are added to cause polymerization or supramolecule synthesis reaction and then removed from the resultant cross-linked polymer or supramolecule to prepare a porous material. The size of pores introduced is dependent on the chemical structure and concentration of the porogens (see Nature Materials 2005, 4, 671; J. Am. Chem. Soc. 1999, 121, 8518).
The fifth method uses microphase separation of block copolymers to prepare polymer nanostructures that have porous morphology, including sphere, lamellae, hexagonally packed cylinder, or zyroid (see Science 2008, 322, 429; Macromolecules 1996, 29, 1091; ACS NANO 2010, 4, 285; NANO Lett. 2009, 9, 4364).
In the sixth method, a polymer colloid of a constant size is three-dimensionally arranged to form polymer colloid crystals and then exposed to an electron beam to obtain a three-dimensionally arranged macroporous polymer structure (see Adv. Mater. 2005, 17(1), 120-125).
The above-described six methods are however impractical for the preparation of porous materials in an industrial scale because most of them require strictly controlled reaction conditions or involve complicated reaction mechanisms. Furthermore, the porous materials prepared by the respective methods are hard to control in pore size and thus limited in their applicable usage.